<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>

</body>
<script type="text/javascript">
    var a;
    console.log(a, typeof a, typeof a === 'undefined', a === undefined);

    var b = 4;
    console.log(b, typeof b);

    var c = 'abc';
    console.log(c, typeof c);

    var d = true;
    console.log(d, typeof d);

    var e = null;
    console.log(e, typeof e);

    var arr = [1, 2, 'abc'];
    console.log(arr, typeof arr, arr instanceof Array, arr instanceof Object);
</script>
</html>